Martin Ullrich – PhD thesis summary

Formation of liposomes for controlled release of reactants and actives

This dissertation thesis deals with the preparation of liposomes and their incorporation into microparticles containing other materials. The primary motivation is a possibility of utilization of such structures for targeted drug delivery. Liposomes are hollow particles with sizes from tens of nanometres to tens of micrometres. Their wall is composed of phospholipids and the chemical composition of this membrane defines the surface charge or sensitivity of liposomes to changes of external conditions. In this thesis, primarily thermo-sensitive liposomes will be used, because their membrane permeability changes significantly when heated.

In the first part of the thesis, a temperature dependent release rate of a fluorescence dye with respect to temperature and cholesterol content is investigated. When heated above 40 °C, it was found, that the membrane permeability is increased more than 1000-times for liposomes from phospholipid dipalmitoyl-phospho-choline (DPPC) with up to 20 mol.% of cholesterol. These liposomes were then immobilized into the calcium alginate gel capsules with a diameter of several millimetres. The preparation conditions for these composites were optimized in order to minimize the premature leakage of the model substance. It was found, that the addition of at least 10 mol.% of cholesterol to the DPPC membranes and decreasing the preparation temperature is a feasible way of keeping the premature leakage of the model substance below 10 % during the preparation process.

The gel particles were scaled down to several tens of micrometres in the next step. The size decrease was achieved by using a piezoelectric ink-jet nozzle. After optimization of the ink-jet setup (voltage, frequency, etc.), the ink-jet generated small droplets of the gel precursor that contained liposomes. The droplets then became solid in a cross-linker solution. Also magnetic nanoparticles were added to the system. They enable contactless radiofrequency heating of the gel particles and generation of heat needed for the permeability increase of the thermo-sensitive membranes of liposomes. The composite microparticles (liposomes, magnetic nanoparticles and hydrogel) capable of on-demand release of encapsulated substances were formed as the result. A similar principle was used for the control of an enzymatic reaction inside the microparticles. The substrate was encapsulated within the liposomes and an enzyme that catalysed the oxidation of the substrate was dispersed in the gel microparticles. The reaction-diffusion processes within the microparticles were mathematically described and the mathematical model for the release of the product was created.

Besides those composite microparticles, the direct aggregation of liposomes into larger agglomerates and stabilization of the agglomerates were also studied. The reason was primarily because of the low volume concentration of liposomes in the gel capsules, and therefore low encapsulated concentrations. The aggregation rate of positively charged liposomes with respect to the sodium chloride concentration was studied by dynamic light scattering. After the preparation of the agglomerates with the required size distribution, the aggregation was terminated by colloidal stabilization of the aggregates by the polypropylene-polyethylene copolymer.